1. Field of the Invention
The present invention relates to electroluminescent and photoluminescent polymer compositions and processes for their preparation and use in, for example, electroluminescent devices such as electroluminescent displays and liquid crystal displays.
2. Description of the Related Art
Electroluminescent (EL) devices are structures which emit light when subject to an applied electric field. The usual model for the physical process in a semiconductor used in this way is through the radiative combination of electron-hole pairs which are injected into the semiconductor from opposite electrodes. Common examples are light-emitting diodes based on Gap and similar III-V semiconductors. Though these devices are efficient and widely used, they are limited in size, and are not easily or economically used in large area displays. Alternative materials which can be prepared over larger areas are also known. Among the inorganic semiconductors, most effort has been directed to ZnS, which has considerable practical drawbacks and primarily poor reliability. The mechanism in ZnS is believed to be one in which acceleration of one type of carrier through the semiconductor under a strong electric field causes local excitation of the semiconductor which relaxes through radiative emission.
Among organic materials, simple aromatic molecules such as anthracene, perylene and coronene are known to show electroluminescence. The practical difficulty with these materials is, as with ZnS, their poor reliability, together with difficulties in deposition of the organic layers and the currentin-jecting electrode layers. Techniques such as sublimation of the organic material suffer from the disadvantage that the resultant layer is soft, prone to recrystallization, and unable to support high temperature deposition of top-contact layers. Techniques such as Langmuir-Blodgett film deposition of suitably-modified aromatics suffer from poor film quality, dilution of the active material, and high cost of fabrication.
Solid-state light-emitting diodes (LEDs) have found widespread application in displays, as well as in a variety of other applications. LEDs are typically fabricated from conventional semiconductors, for example, gallium arsenide (GaAs), typically doped with aluminum, indium, or phosphorus. Using this technology, however, it is very difficult to make large area displays. In addition, the LEDs made of these materials are typically limited to the emission of light at the long wavelength end of the visible spectrum. For these reasons, there has been considerable interest for many years in the development of suitable organic materials for use as the active (light-emitting) components of LEDs. The utilization of semiconducting organic polymers (i.e., conjugated polymers) in the fabrication of LEDs expands the use of organic materials in electroluminescent devices with the possibility of significant advantages over existing LED technology.
Among the most recent discoveries was the discovery that conjugated polymers are particularly well suited for this purpose in that they provide excellent charge transport characteristics and useful quantum efficiencies for luminescence. Conjugated polymers are an important class of light emitting polymers for electroluminescent (EL) devices. A conjugated polymer is a polymer which possesses a delocalised xcfx80-electron system along the polymer backbone; the delocalised xcfx80-electron system confers semiconducting properties to the polymer and gives it the ability to support positive and negative charge carriers with high mobilities along the polymer chain. Such polymers are discussed, for example, by R. H. Friend in Journal of Molecular Electronics 4 (1988) January-March, No. 1, pages 37 to 46. The most popular of the materials suitable for this use is poly (phenylene vinylene) (PPV) which is capable of being prepared in the form of a high quality film which evidences strong photoluminescence in a band centered near 2.2 eV.
There are two principal approaches to the fabrication of conjugated polymer thin films, namely, the precursor approach and the side chain approach. The former relies on the preparation of a soluble precursor polymer which can be cast into thin films. The precursor polymer can then be converted to the final conjugated polymer films through solid-state thermo- or photo-conversion. Friend et al., refers to EL devices based on poly(p-phenylene vinylene) (PPV) thin films derived from a sulfonium precursor route, see, e.g., U.S. Pat. No. 5,247,190, issued Sep. 21, 1983, to Friend et al. Friend et al. also refers to an electroluminescent device having a semiconductor layer in the form of a thin dense polymer film including at least one conjugated polymer, a first contact layer in contact with a first surface of the semiconductor layer, and a second contact layer in contact with a second surface of the semiconductor layer. The polymer film of the semiconductor layer has a sufficiently low concentration of extrinsic charge carriers that, on applying an electric field between the first and second contact layers across the semiconductor layer so as to render the second contact layer positive relative to the first contact layer, charge carriers are injected into the semiconductor layer and radiation is emitted from the semiconductor layer. The polymer film can be a poly(p-phenylene vinylene) wherein the phenylene ring may optionally carry one or more substituents each independently selected from alkyl, alkoxy, halogen or nitro.
What is needed is a dendritic polymer, specifically a poly(phenylene vinylene) polymer for use in an electroluminescent device, which polymer is self-ordering in the solid state, has thermotropic liquid crystalline phases, enhanced photoconductivity, better charge transport capability, and improved polarized light emission over prior poly(phenylene vinylene) polymers.
The present invention relates to dendritic polymer such as a poly(phenylene vinylene) polymer having dendritic sidechains. The dendritic sidechains enhance the main-chain poly(phenylene vinylene) separations in the solid state. The poly(phenylene vinylene) polymers according to the present invention may be synthesized using, e.g., the Heck polymerization method to achieve a resultant weight-average molecular weight of from about 20,000 to about 60,000 Daltons. The polymers are self-ordering in the solid state, have thermotropic and lyotropic liquid crystalline phases and show enhanced photoluminescence efficiency and polarized light emission over prior poly(phenylene vinylene) polymers.
The present invention also provides processes for preparing soluble poly(phenylene vinylene) polymers having dendritic side chains, which polymers are self-ordering in the solid state, have thermotropic liquid crystalline phases, enhanced photoconductivity, better charge transport capability, and improved polarized light emission.
The above and other advantages and features of the invention will be more clearly understood from the following detailed description together with the accompanying drawings.